Capacitive sensor filtering apparatus, method, and system

ABSTRACT

A transform is used to transform raw capacitive sensor data from the time domain to the frequency or sequency domain. The transformed data falls into several signal bins. The transformed data in at least one of the signal bins is analyzed to determine whether a touch event or release event has occurred.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of U.S. Provisional Patent ApplicationNo. 61/867,197, filed on Aug. 19, 2013, and incorporates the disclosurethereof in its entirety.

BACKGROUND

Known capacitive sensors typically include a sensor electrode disposedon a dielectric substrate and a control circuit connected to the sensorelectrode. A panel or substrate made of glass, plastic, or anothersuitable dielectric material may overlie the sensor electrode and definea touch surface overlying the sensor electrode.

The control circuit provides an excitation voltage to, and therebygenerates an electric field about, the sensor electrode. This electricfield establishes a capacitance, sometimes referred to as a parasiticcapacitance, from the sensor electrode to ground or another referencepotential (the terms “ground” and “other reference potential” may beused interchangeably herein). Introduction of a stimulus, for example, auser's finger, to or near the sensor electrode or corresponding touchsurface establishes an additional capacitance, sometimes referred to asa finger capacitance, from the electrode, through the finger, to ground,thereby changing the overall sensor electrode-to-ground capacitance.

The control circuit also detects and monitors the sensorelectrode-to-ground capacitance. The control circuit uses this data inconjunction with predetermined criteria to deem whether or not a touchevent or proximity event has occurred. The control circuit could beconfigured to deem a touch event to occur when a stimulus touches thesensor electrode or corresponding touch surface. The control circuitcould be configured to deem a proximity event to occur when the stimulusis near but not touching the sensor electrode or the touch surface. Thecontrol circuit could be configured to deem touch or proximity events tooccur under other circumstances, as well, as would be understood by oneskilled in the art.

The control circuit could be configured to deem whether or not a touchevent or proximity has occurred by comparing the sensorelectrode-to-ground raw capacitance at any given time, or for any givensample of raw capacitive data points, to a predetermined thresholdand/or by comparing the raw capacitance to a baseline raw capacitance.The baseline raw capacitance typically would be a time-averaged measureof raw counts when no stimulus is near or proximate the sensorelectrode.

For example, the control circuit could be configured to deem a touchevent or proximity event to occur when the raw count exceeds or fallsbelow a predetermined threshold. Alternatively, the control circuitcould be configured to deem a touch event or proximity event to occurwhen the raw count deviates from the baseline raw count by at least apredetermined amount or difference.

In some embodiments, the control circuit could be configured torecognize and distinguish between both touch and proximity events. Forexample, the control circuit could be configured to deem a proximityevent to occur when the raw count exceeds or falls below a firstpredetermined threshold or when the raw count deviates from the baselineraw count by at least a first predetermined difference. The controlcircuit also could be configured to deem a touch event to occur when theraw count exceeds or falls below a second predetermined threshold orwhen the raw count deviates from the baseline raw count by at least asecond predetermined difference. Typically, the raw count would deviatefrom the baseline by a greater amount in response to touch of thestimulus to the sensor electrode or touch surface compared to mereproximity of the stimulus to the sensor electrode or touch surface. Assuch, the first predetermined threshold or difference typically wouldlie between the baseline and the second predetermined threshold ordifference.

Conventional capacitive touch systems may have certain drawbacks. Forexample, the control circuits typically need to be tuned for aparticular touch scenario. For example, they typically need to be tunedto detect touch by or proximity of a bare finger versus touch by orproximity of a gloved finger. Touch by or proximity of a bare fingertypically would result in a greater finger capacitance and, therefore, agreater change in raw capacitance versus the baseline raw capacitance,than would touch by or proximity of a gloved finger. Indeed, the changein raw capacitance resulting from touch by a bare finger could be twice(or more or less) than the change in raw capacitance resulting fromtouch by a gloved finger.

As such, a system that is tuned to reliably detect touch by or proximityof a bare finger might not be sufficiently sensitive to detect touch byor proximity of a gloved finger. That is, touch by a gloved finger mightnot yield a change in raw counts exceeding the touch or proximitythreshold. Although a system can be tuned to respond to touch by agloved finger, a system so tuned might be overly sensitive to a barehand such that a touch is deemed to have occurred when the bare hand ismerely proximate but not touching the touch surface. A system so tunedalso might be unacceptably susceptible to falsely “detect” touch whentouch has not occurred because of noise or the presence of water, othercontaminants, or spurious stimuli proximate or in contact with thesensing electrode or touch surface.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a capacitive sensor 10 including a sensing electrode12 and ground plane 14 disposed on a circuit carrier 16 attached to atouch panel 18 defining a touch surface 20 corresponding to sensingelectrode 12;

FIG. 2A illustrates the capacitance about sensor 10 in the steady statein the absence of a stimulus;

FIG. 2B illustrates the capacitances about sensor 10 with a stimulus Stouching touch surface 20;

FIG. 3 is a graph showing the raw capacitive values associated with acapacitive touch sensor in untouched and touched states as a function oftime;

FIG. 4 illustrates the transformation of raw capacitive values for aneight sample sequence from the time domain to the sequency domain usinga Walsh Hadamard Transform;

FIG. 5 illustrates the sequency bins yielded by the Walsh HadamardTransform illustrated in FIG. 4;

FIG. 6 illustrates capacitive touch in the sequency domain as a functionof the eight sequency bins illustrated in FIG. 3;

FIG. 7 illustrates the sequency domain distribution curve associatedwith sequency bin 0;

FIG. 8 illustrates the sequency domain distribution curve associatedwith sequency bin 1;

FIG. 9 illustrates the sequency domain distribution curve associatedwith sequency bin 1 as illustrated in FIG. 8 superimposed upon the timedomain signal illustrated in FIG. 3;

FIG. 10 illustrates the sequency domain distribution curve associatedwith sequency bin 2;

FIG. 11 illustrates the sequency domain distribution curve associatedwith sequency bin 3;

FIG. 12 illustrates the sequency domain distribution curve associatedwith sequency bin 4;

FIG. 13 illustrates the sequency domain distribution curve associatedwith sequency bin 5;

FIG. 14 illustrates the sequency domain distribution curve associatedwith sequency bin 6;

FIG. 15 illustrates the sequency domain distribution curve associatedwith sequency bin 7;

FIG. 16 illustrates the sequency domain distribution curve associatedwith sequency bins 1-7 superimposed on each other over the course of ano-touch condition followed by a touch event, which is followed by ahold event, which is followed by a release event;

FIG. 17 illustrates the sequency domain distribution curves associatedwith sequency bins 1 and 3 and a distribution curve showing thedifference between the distribution curves associated with bins 1 and 3at each sample point superimposed on each other during touch and releaseevents;

FIG. 18 illustrates the sequency domain distribution curve associatedwith sequency bin 1 during a slow touch event;

FIG. 19 illustrates the sequency domain distribution curve associatedwith sequency bin 1 and the absolute summation of the sequency domaindistribution curves associated with sequency bins 4-7 superimposed oneach other during touch and release events;

FIG. 20 illustrates the sequency domain distribution curve associatedwith sequency bin 1 and the absolute summation of the sequency domaindistribution curves associated with sequency bins 4-7 superimposed oneach other during touch and release events;

FIG. 21 illustrates the sequency domain distribution curves associatedwith sequency bins 1-3 and the absolute summation of the sequency domaindistribution curves associated with sequency bins 4-7 superimposed oneach other during touch and release events;

FIG. 22 illustrates the sequency domain distribution curves associatedwith sequency bins 1-3 and the absolute summation of the sequency domaindistribution curves associated with sequency bins 4-7 superimposed oneach other during touch, extended hold, and release events;

FIG. 23 illustrates frequency domain distribution curves showing realdata associated with frequency bins 0-7;

FIG. 24 illustrates frequency domain distribution curves showing realdata associated with frequency bins 1-3;

FIG. 25 illustrates frequency domain distribution curves showing realdata associated with frequency bins 1-3 and indicates touch events;

FIG. 26 illustrates frequency domain distribution curves showingimaginary data associated with frequency bins 1-3;

FIG. 27 illustrates frequency domain distribution curves showingimaginary data associated with frequency bins 1-7;

FIG. 28 illustrates frequency domain distribution curves showingmagnitude data associated with frequency bins 1-3;

FIG. 29 illustrates frequency domain distribution curves showingmagnitude data associated with frequency bins 1-7;

FIG. 30 illustrates an alternate electrode structure; and

FIG. 31 illustrates another alternate electrode structure.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a typical capacitive sensor 10. Sensor 10 includes asensor electrode 12 and a ground plane 14 disposed on a dielectriccircuit carrier substrate 16. Circuit carrier substrate 16 typicallywould be, for example, a piece of glass or plastic, a printed wiringboard or a flexible circuit carrier. A touch surface substrate 18,typically a piece of glass, plastic, or other suitable dielectricmaterial, overlies and is attached to circuit carrier substrate 16.Touch surface substrate 18 defines a touch surface 20. Touch surface 20overlies and generally is aligned with sensor electrode 12. In someembodiments, touch surface substrate 18 could be omitted, and touchsurface 20 could be similarly defined by circuit carrier substrate 16,for example, the surface of circuit carrier substrate opposite thesurface upon which the sensor electrode is disposed. A control circuit(not shown) is electrically connected to sensor electrode 12. Thecontrol system may include a microprocessor, means for exciting (forexample, providing an excitation voltage to) sensor electrode 12, andmeans for detecting capacitance from sensor electrode 12 to ground oranother reference potential (the terms “ground” and “referencepotential” may be used interchangeably herein), among other components.

In operation, the control circuit excites, and thereby generates anelectric field represented by electric field lines E about, sensorelectrode 12. This electric field E couples to ground through groundplane 14 and thereby generates a parasitic capacitance C_(p) betweensensor electrode 12 and ground. This phenomenon is illustrated in FIG.2A. In the steady state, in the absence of a stimulus proximate ortouching touch surface 20 or sensor electrode 12, this parasiticcapacitance has a time-averaged baseline value C_(p).

When a stimulus S, for example, a user's finger or other conductiveobject, is introduced proximate sensor electrode 12, an additionalcapacitance, sometimes referred to as a stimulus capacitance, C_(s) isestablished between sensor electrode 12 and ground through the stimulusS. This phenomenon is shown in FIG. 2B, wherein stimulus S is showntouching touch surface 20, which is proximate sensor electrode 12. (Thefield lines E are not shown in FIG. 2B for clarity.) The stimuluscapacitance C_(s) would reach a theoretical maximum if stimulus S wereto contact sensor electrode 12. The stimulus capacitance reaches apractical maximum when the stimulus comes into contact with touchsurface 20. The stimulus capacitance C_(s) would have a lesser valuewhen the stimulus S is proximate, but not in contact with, touch surface20 than it would when the stimulus is in contact with touch surface 20.

In typical applications, the system capacitance (including primarily theparasitic capacitance C_(p)) is measured and processed in terms of rawcounts, as would be understood by one skilled in the art. In the steadystate, with no stimulus proximate or touching touch surface 20, the rawcount has a time-averaged baseline value. Depending on the conventionused, the raw count increases or decreases from the baseline value toanother value when a stimulus is introduced near or into contact withtouch surface 20. (The drawings illustrate the raw count decreasing fromthe baseline in response to such introduction of a stimulus andcorresponding distribution curves for the raw count transformed into thesequency and frequency domains. In embodiments wherein the raw countincreases from the baseline in response to such introduction of astimulus, the curves would be inverted.)

FIG. 3 illustrates the response of a typical capacitive sensor to fourdistinct touch and release events as observed in the time domain. Thehorizontal axis of FIG. 3 represents units of time or samples, and thevertical axis represents raw counts. The vertical axis could representother units indicative of capacitive sensor operation, as would beunderstood to one skilled in the art.

The capacitive sensor illustrated in FIG. 3 has a baseline raw count,that is, a count with no stimulus proximate or touching sensor electrode12 or touch surface 20, of about 440. This raw count represents theparasitic capacitance Cp. At about sample 18, the raw count begins todrop from the baseline value. This drop in counts corresponds to theapproach of stimulus S to sensor electrode 12 or touch surface 20 andthe establishment of an additional capacitive coupling, for example,stimulus capacitance C_(s), from sensor electrode 12, through stimulusS, to ground, as discussed above. As stimulus S gets closer to sensorelectrode 12 or touch surface 20, the magnitude of the capacitivecoupling through stimulus S increases, and the value of the raw countdecreases.

At about sample 22, the raw count bottoms out at a first depressed valueof about 395-400 counts. This sample corresponds to the initial touch ofstimulus S to sensor electrode 12 or touch surface 20.

The raw count remains at about the first depressed value until aboutsample 30. The samples from sample 22 to sample 30 correspond to themaintained contact of stimulus S to sensor electrode 12 or touch surface20.

At about sample 30, the raw count begins to increase from the firstdepressed value. This increase in counts corresponds to the withdrawalof stimulus S from sensor electrode 12 or touch surface 20 and thereduction of the additional capacitive coupling from sensor electrode12, through stimulus S, to ground, as discussed above. As stimulus Sgets farther from sensor electrode 12 or touch surface 20, the magnitudeof the capacitive coupling through stimulus S decreases, and the valueof the raw count increases.

At about sample 33, the raw count reverts to the baseline. The raw countremains at about the baseline until about sample 42. The samples fromsample 33 to sample 42 correspond to the withdrawal of stimulus S fromsensor electrode 12 or touch surface 20 by a sufficient distance suchthat sensor electrode 12 is no longer capacitively coupled (or isinsignificantly capacitively coupled) through stimulus S to ground.

At about sample 42, the raw count again begins to drop from thebaseline. At about sample 50, the raw count bottoms out at a seconddepressed value of about 390-395 counts. At about sample 54, the rawcount begins to increase from the second depressed value. At aboutsample, 59, the raw count reverts to the baseline. This reduction,bottoming out, hold, increase, and reversion to the baseline represent asecond approach, contact, hold, and withdrawal of stimulus S asdescribed above.

A similar cycle is repeated two more times, as would be recognized byone skilled in the art.

FIG. 3 illustrates a touch threshold of about 410 counts (or 30 countsbelow the baseline) for the capacitive sensor represented therein. Assuch, a touch event is deemed to occur when the raw count falls below410 counts. The touch threshold of 410 counts is provided forillustration only. The touch threshold could be selected as desired atany suitable value higher or lower than 410 counts for a particularapplication, as would be understood by one skilled in the art.

The touch threshold could be selected so that a touch event is deemed tooccur when touch surface 20 is actually touched by a bare finger.Alternatively, the touch threshold could be selected so that a touchevent is deemed to occur when touch surface 20 is actually touched by agloved finger. Further, the touch threshold could be selected so that atouch event is deemed to occur when a bare or gloved finger isproximate, but not touching touch surface 20 (sometimes referred to as a“proximity event”). As discussed above, the additional capacitanceresulting from proximity or touch of a bare finger to touch surface 20typically would be substantially greater than the additional capacitanceresulting from proximity or touch of a gloved finger to touch surface20.

Systems, such as the one described above, that rely strictly ondifferences in raw count in order to deem whether or not a touch eventhas occurred have certain shortcomings. For example, a system that istuned to detect touch by a bare finger might not be able to readilydetermine touch by a gloved finger because the additional capacitanceresulting from proximity of the gloved finger to sensor electrode 12 ortouch surface 20 is insufficient to alter the raw counts in excess ofthe touch threshold selected to detect touch by a bare finger. Althoughin some instances such a system could be tuned to detect touch by agloved finger, the system so tuned might by unduly sensitive to touch orproximity by a bare finger. For example, a system that is tuned to deema touch event to occur when a gloved finger touches touch surface 20might also deem to a touch to occur when a bare finger is merelyproximate, and not touching, touch surface 20.

In essence, the challenge lies in selecting a threshold low enough todetect touch by a gloved finger (which results in a relatively smalladditional capacitance and, therefore, a relatively small effect on thesystem capacitance) but at the same time high enough to not deem a touchto have occurred as a result of mere proximity (but not touch) of astimulus to the touch surface, or as a result of noise, crosstalk,interference or other parasitic phenomena.

This challenge may be met by transforming the raw capacitance signalsfrom the time domain to the frequency domain using any suitabletransform technique, and then evaluating the transformed signals in thetransformed signal bins. A Fast Fourier Transform (“FFT”) is onesuitable form of transform technique. An FFT, however, is relativelymath intensive. For example, it involves multiplication and processingof complex numbers. Performing an FFT quickly enough to be useful inanalyzing capacitive sensor signals and determining whether a touchevent has occurred in real time requires the use of a relativelypowerful, and relatively expensive, processor.

Alternatively, this challenge may be met by transforming the rawcapacitance signals from the time domain to the sequency domain using,for example, a Walsh-Hadamard transform or a Fast Walsh-Hadamardtransform, and then evaluating the transformed signals in thetransformed signal bins. A Walsh-Hadamard transform can be performedusing only basic addition and subtraction, and it does not involve theuse of complex numbers. As such, performing a Walsh-Hadamard transformrequires relatively little computing power compared to performing a FastFourier Transform (FFT). Indeed, in at least some applications, aWalsh-Hadamard transform could be performed using the microprocessorthat might be included as part of a conventional capacitive sensor'scontrol system. Also, because a Walsh-Hadamard transform involvesrelatively little computation, and because the computation is relativelysimple, a Walsh-Hadamard transform can be performed relatively quickly.

FIG. 4 illustrates a representative eight point sample of capacitivesignal data as a function of time. The eight point rolling sampleincludes the most current sample, as well as the seven previous samples.(The most current sample shown in FIG. 4 corresponds to sample 1 of FIG.3; the seven previous samples are not shown in FIG. 3.) The most currentsample is in the rightmost position of the string illustrated in FIG. 4.The second-most current sample is immediately to the left of the mostcurrent sample. The third-most current sample is immediately to the leftof the second-most current sample, and so on.

FIG. 4 also illustrates transformation of the eight point sample ofcapacitive signals from the time domain to the sequency domain bymultiplying the eight point sample by an 8×8 Walsh matrix. Thetransformation yields eight transformed signals, as would be understoodby one skilled in the art, and as discussed further below. The eighttransformed signals fall into eight corresponding signal bins in thesequency domain. FIG. 4 illustrates the signal bins arranged in Hadamardorder, which represents the sequency data in directly output from thetransform.

FIG. 5 illustrates how the signal bins could be arranged in sequencyorder, that is, in the order of increasing sequency. As best shown inFIG. 5, the left-most signal bin is defined as sequency bin 0. Thesecond sequency bin from the left is defined as sequency bin 7. Thethird sequency bin from the left is defined as sequency bin 3. Thefourth sequency bin from the left is defined as sequency bin 4. Thefifth sequency bin from the left is defined as sequency bin 1. The sixthsequency bin from the left is defined as sequency bin 6. The seventhsequency bin from the left is defined as sequency bin 2. The eighthsequency bin from the left is defined as sequency bin 5.

Multiplying the eight point sample by the 8×8 Walsh matrix involvesadding and subtracting the data values for each point in the sampleaccording to the signs in each row of the matrix. For example, the valueof sequency bin 0 is computed by adding together the values of eachpoint of the eight point string as dictated by the first row of theWalsh matrix. The value of sequency bin 7 is computed by adding thevalues of the first, third, fifth and seventh points and subtracting thevalues of the second, fourth, sixth and eighth points of the eight pointstring as dictated by the second row of the Walsh matrix. The value ofsequency bin 3 is computed by adding the values of the first, second,fifth and sixth points and subtracting the values of the third, fourth,seventh and eighth points of the string as dictated by the third row ofthe Walsh matrix. The value of sequency bin 4 is computed by adding thevalues of the first, fourth, fifth and eighth points and subtracting thevalues of the second, third, sixth and seventh points as dictated by thefourth row of the Walsh matrix. The values of the remaining sequencybins are bins are computed in a similar manner, as dictated by thecorresponding rows of the Walsh matrix, and as would be understood byone skilled in the art.

Once a particular eight point sample has been transformed, as discussedabove, the next eight point sample is transformed in a similar manner.Once the next eight point sample has been transformed, the followingeight point sample is transformed, and so on. As such, the methodinvolves substantially continuous transformation of an eight pointrolling sample, with each eight point rolling sample including the thenmost current sample and the seven immediately preceding samples.

FIG. 6 illustrates the capacitive signal data of FIG. 3 after havingbeen transformed, using an eight point rolling sample window to yieldthe signal expanded into the eight sequency bins, as discussed above.

FIG. 7 illustrates a distribution curve for the signal components insequency bin 0. This distribution curve represents the area under thecurve of FIG. 3 (which represents the raw capacitive count at eachsample in the time domain) for each eight point rolling sample uponwhich the Walsh-Hadamard transform has been performed. For example, thecount value of 3528 at sample point 1 of FIG. 7 represents the sum ofthe counts for the rolling sample including sample 1 of FIG. 3 and theseven immediately preceding samples (which are not shown in FIG. 3).Similarly, the count value of 3527 at sample point 2 of FIG. 7represents the sum of the counts for the rolling sample including sample2 of FIG. 3 and the seven immediately preceding samples (six of whichare not shown in FIG. 3). Further, the count value of 3508 at samplepoint 8 of FIG. 7 represents the sum of the counts for the rollingsample including sample 8 of FIG. 3 and the seven immediately precedingsamples (that is, samples 1-7), and so on.

The distribution curve of FIG. 7 is similar to the curve of FIG. 3 inthat it shows a baseline count (about 3500 counts), decreasing counts(down to about 3200-3300 counts) in response to a touch, and increasingcounts in response to release of the touch. More specifically, thedownward sloping portions of the curve represent the system response toa touch, and the upwardly sloping portions of the curve represent thesystem response to release of the touch.

According to a first method, the data of sequency bin 0 could be used todetermine whether a touch event has occurred in a manner similar to thatin which raw capacitance data is used to determine whether a touch eventhas occurred in the time domain. More particularly, a thresholdrepresenting a difference in counts from the baseline could beestablished and a touch could be deemed to have occurred when the countsare less than the threshold (or otherwise deviate from the baseline bymore than a threshold amount). Similarly, a release could be deemed tohave occurred when the counts return to a value above the threshold (orto the baseline value or a value that deviates from the baseline by lessthan a threshold amount (which may the same as, greater than, or lessthan the threshold mound used to determine whether a touch hasoccurred)). In one example, the threshold could be 240 counts, whichrepresents eight times the thirty count time domain thresholdillustrated in FIG. 3. The multiplier of eight used here is a functionof the eight point sample size. In other embodiments, other thresholdscould be established, which thresholds could, but need, be a function ofthe time domain threshold or the sample size used in performing theWalsh-Hadamard transform as discussed herein. Because this means ofdetermining whether a touch event has occurred is based on eightsamples, rather than a single sample as is the case in the time domain,this means has a better signal-to-noise ratio than the time domainmeans.

FIG. 8 illustrates a distribution curve for sequency bin 1. Thisdistribution curve shows a count hovering about a baseline of about zerocounts, a positive excursion to a maximum of about 150 counts, anegative excursion to a minimum of about −150 counts, and a positiveexcursion back to the baseline. This or a substantially similar cycle isrepeated four times, corresponding to the four touch and release eventsillustrated in the time domain in FIG. 3.

In FIG. 8, the baseline about zero represents the untouched condition,each rise in counts from the baseline to the positive peak representsapproach of a stimulus to a sensor electrode or touch surface, eachpositive peak and fall from the positive peak to the negative peakrepresents touch of the stimulus, and each negative peak and the risefrom the negative peak to the baseline represents release of thestimulus. As such, FIG. 8 represents four taps and releases of stimulusS to and from sensor electrode 12 or touch surface 20.

FIG. 9 illustrates the distribution curve of FIG. 8 superimposed ontothe time domain signal of FIG. 3.

With reference to FIG. 8, according to a second method, the data ofsequency bin 1 could be used in several ways to determine whether atouch event has occurred. For example, a touch could be deemed to haveoccurred when the previous bin sample is greater than the current binsample, preferably by at least a predetermined amount or threshold. Withreference to FIG. 8, a touch could be deemed to have occurred at samplepoint 27 because the count at previous sample point 26 is greater thanthe count at sample point 27. (The touch event would actually haveoccurred at sample point 26, where the raw count reaches a maximum.)Similarly, a touch would not be deemed to have occurred at sample point26 because the count at the previous sample point, namely, sample point25, is not greater than the count at sample point 26.

According to the second method, the data of sequency bin 1 could be usedto determine that a release event has occurred when the count has gonenegative and the current sample is greater than the previous sample.With reference to FIG. 8, a release could be deemed to have occurred atsample point 35 because the count is negative and the count at samplepoint 34 is greater than the count at immediately previous sample point33. (The release would actually have occurred at sample point 33, wherethe count reaches a minimum.)

With continued reference to FIG. 8, according to a third method, a touchcould be deemed to have occurred when the counts of sequency bin 1 crosszero in the positive direction and then cross zero in the negativedirection. A release could be deemed to have occurred when the counts ofsequency bin 1 cross zero in the negative direction and then cross zeroin the positive direction.

The flat portion of the curve of FIG. 8 between the four touch andrelease events (that is, between adjacent negative peaks and positivepeaks) represents a no-touch condition. Though the phenomenon is notillustrated in FIG. 8, a held touch could be evidenced by a count aboutzero and between a leading positive peak and a following negative peak.

With continued reference to FIG. 8, according to a fourth method, atouch could be deemed to have occurred when the value of the currentsequency bin 1 signal and the value of the previous sequency bin 1signal differ by more than a predetermined positive threshold amount.This phenomenon may be represented by a positive slope of the curvebetween samples points of sequency bin 1. A release could be deemed tohave occurred when the value of the current sequency bin 1 signal andthe value of the previous sequency bin 1 signal differ by more than apredetermined negative threshold amount. This phenomenon may berepresented by a negative slope of the curve between samples points ofsequency bin 1.

FIG. 10 illustrates a distribution curve for sequency bin 2. Thisdistribution curve shows counts hovering about a baseline of about zerocounts, a negative excursion to about −55 counts, a positive excursionto about 60 counts, a second negative excursion to about zero counts, asecond positive excursion to about 70 counts, a third negative excursionto about −70 counts, and a third positive excursion to about thebaseline. This or a substantially similar cycle is repeated four times,corresponding to the four touch and release events illustrated in thetime domain in FIG. 3.

FIG. 11 illustrates a distribution curve for sequency bin 3. Thisdistribution curve shows a count hovering about a baseline of about zerocounts, a positive excursion to about 75 counts, a negative excursion toabout ten counts, a second positive excursion to about 65 counts, asecond negative excursion to about −70 counts, a third positiveexcursion to about −15 counts, a third negative excursion to about −70counts, and a fourth positive excursion to about zero counts. This or asubstantially similar cycle is repeated four times, corresponding to thefour touch and release events illustrated in the time domain in FIG. 3.

FIGS. 12-15 illustrate distribution curves for sequency bins 4-7,respectively. Each of these figures illustrates a count hovering about abaseline of about zero counts, and a number of positive and negativeexcursions thereafter.

FIG. 16 illustrates the signals of sequency bins 1-7 superimposed oneach other, and identifies a no-touch condition, a touch event, a holdevent, and a release event.

The signal data from the higher sequency bins (bins 2-7) could be usefulin determining whether a touch event has occurred. For example,according to a fifth method, a touch may be deemed to have occurred whenthe signals in bins 1, 2, 3, 7 and 6 are above zero and the signals inbins 4 and 5 are below zero. Similarly, a release may be deemed to haveoccurred when the signals in bins 1, 2, 3, 7 and 6 are below zero andthe signals in bins 4 and 5 are above zero. These situations areillustrated in FIG. 16.

Also, according to a sixth method, a touch may be deemed to haveoccurred when the difference between the values of bins 1 and 3 isgreater than a predetermined threshold, the value of bin 3 is less thananother predetermined threshold, and the value of bin 1 is greater thanzero. The predetermined thresholds used in the touch analysis may be thesame or different and either may be greater or lesser than the other. Arelease may be deemed to have occurred when the difference between thevalues of bins 1 and 3 is greater than a predetermined threshold, thevalue of bin 3 is less than another predetermined threshold, and thevalue of bin 1 is less than zero. The predetermined thresholds used inthe release analysis may be the same or different and either may begreater or lesser than the other. The predetermined thresholds used inthe touch analysis may be the same as or different from thepredetermined thresholds used in the release analysis. This situation isillustrated in FIG. 17.

FIG. 18 illustrates the signals of sequency bin 1 and identifies a slowtouch, a hold and a normal (rather than slow) release. “Slow” as used inthis context refers to actions that are relatively gradual as opposed torelatively abrupt. For example, “slow touch” and “slow release” mayrefer, respectively, to a gradual approach of a stimulus to and gradualremoval of a stimulus from touch surface 20 or sensor electrode 12,rather than a quick, relative abrupt tap and release of a stimulusthereto.

Detection of a slow touch or release event using capacitive data in thetime domain may be difficult based on the manner in which the capacitivereference that is used in conjunction with a capacitive threshold inorder to determine whether a touch or proximity event has occurredtypically is established, used, and/or maintained. Conventionalcapacitive systems may include means to continuously adjust thecapacitive reference to compensate for temperature fluctuations, noise,and contamination that may build up on the touch surface or sensorelectrode over time, all of which can affect and alter the baselinecapacitance, i.e., the raw capacitance in the absence of a stimulusproximate the touch surface or sensor electrode.

For example, airborne contaminants may become deposited on and build upupon the touch surface over time. Such contaminants can cause thebaseline capacitance to increase or decrease. If the baselinecapacitance decreases, the additional capacitance provided byintroduction of a stimulus to the touch surface might be insufficient toexceed the raw capacitance threshold so that a touch event is deemed tohave occurred. For example, the baseline capacitance at a given timemight be 400 counts, and the capacitive reference might be set at 400raw counts based on this baseline capacitance. Introduction of astimulus might increase the raw capacitance by 40 counts. A designermight select a touch threshold of 435 counts so that a touch is deemedto occur in response to introduction of the stimulus, even if thebaseline capacitance fluctuates by a few counts from the initialbaseline capacitance of 400 counts. If the baseline capacitance were todrop to 390 counts, however, and introduction of the stimulus were toincrease the raw capacitance by 40 counts to a total of 430 counts, thetouch threshold would not be met or exceeded, and a touch would not bedeemed to occur.

The converse result could occur if the baseline capacitance were toincrease due to such effects. More specifically, if the baseline were toincrease to 420 counts, an increase in capacitance of as little as 16counts would yield a total capacitance in excess of the touch threshold.As such, the system could deem a touch event to have occurred when astimulus is brought near the touch surface or sensor electrode, but notas close as the designer might have intended in order for the system todeem a touch event to have occurred.

Conventional systems may compensate for such effects by dynamicallyadjusting the reference capacitance in response to time-averagedfluctuations in the baseline capacitance. For example, the system may beconfigured to recognize a slowly increasing or decreasing baselinecapacitance over time and dynamically adjust the reference capacitanceand touch threshold accordingly, as would be understood by one skilledin the art. Such dynamic reference adjustment systems, however, mightnot be able to distinguish between “normal” fluctuations in the baselineand slow increases or decreases in raw capacitance resulting from a slowbut deliberate introduction of a stimulus proximate the touch surface orsensor electrode. As such, a conventional dynamic reference adjustmentsystem might continuously adjust the reference capacitance up or down asa consequence of the slow introduction of a stimulus such that thesystem does not recognize or respond to a legitimate touch event. Putanother way, rather than recognizing a steadily increasing or decreasingraw count as resulting from an approaching stimulus, the system mayconsider the change in raw count as resulting from the effects oftemperature, noise, or contaminants, and it may adjust the reference andthreshold accordingly, such that the threshold is not exceeded inresponse to the stimulus.

According to a seventh method, slow touch and release events can bedetected in the sequency domain by integrating transformed data overtime as described further below and comparing the integrated value tocorresponding thresholds to determine whether a touch or release eventhas occurred. More particularly, a slow touch event can be detected byintegrating the transformed values of successive samples in sequency bin1 having transformed values greater than zero, resetting the integratedvalue to zero in response to a sample having a transformed value lessthan zero, and deeming a touch event to have occurred if and when theintegrated value exceeds a predetermined threshold. Similarly, a slowrelease event can be detected by integrating the transformed values ofsuccessive samples in sequency bin 1 having transformed values less thanzero, resetting the integrated value to zero in response to a samplehaving a transformed value above zero, and deeming a touch event to haveoccurred if and when the absolute value of the integrated value exceedsa predetermined threshold.

For example, with reference to FIG. 18, the transformed data from sample1 to sample 12 fluctuates positively and negatively about (above andbelow) the baseline of zero counts. The values of successive sampleshaving positive values are added (integrated) and the resultingintegration is reset to zero when a sample having a negative value isencountered. Similarly, the values of successive samples having negativevalues are integrated and the resulting integration is reset to zerowhen a sample having a positive value is encountered.

More specifically, from sample 1 to sample 8, the transformed values arepositive and are added (integrated). At sample 9, the transformed valueis negative, and the integration is reset to zero. Because theintegration was reset to zero before the threshold was achieved, notouch event is deemed to have occurred between samples 1 and 8.

From sample 9 to sample 11, the transformed values are negative and areadded. At sample 12, the transformed value goes positive and theintegration is reset to zero. Because the integration was reset to zerobefore the threshold was achieved, no release event is deemed to haveoccurred between samples 9 and 11.

At sample 12, the transformed values of successive samples begin toincrease and remain above zero, yielding integration values as set forthin the following chart:

Sample 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 Count 1 2 3 43 3 3 4 6 6 7 6 5 6 5 5 5 Integration 1 3 6 10 13 16 19 23 29 35 42 4853 59 64 69 74 Sample 29 30 31 32 33 34 35 36 37 38 39 40 41 42 Count 33 3 4 7 8 9 9 8 11 15 18 21 19 Integration 77 80 83 87 94 102 111 120128 139 154 172 193 212

At sample 42, the integration of successive samples reaches 212 counts.Assuming for the sake of illustration that the touch threshold is set at200 counts, the integration at sample 42, therefore, exceeds the touchthreshold and a touch is deemed to have occurred at sample 42 and notearlier. At sample 43, the transformed values of successive samplesbegins to decrease. At sample 58, the transformed value goes negative,and the integration is reset to zero. Between sample 59 and sample 95,the transformed values fluctuate about the baseline of zero such thatthe integration is frequently reset to zero before a threshold isreached. As such, between samples 59 and 95, neither a touch event nor arelease event is deemed to have occurred.

At sample 96, the transformed values of successive samples begin todecrease from zero and remain below zero, yielding integration values asset forth in the following chart:

Sample 96 97 98 99 100 101 Count −1 −13 −32 −52 −71 −68 Integration −1−14 −46 −98 −169 −237

At sample 101, the absolute value of the integration of successivesamples reaches 237 counts. Assuming for the sake of illustration thatthe release threshold is set at 200 counts, the integration at sample101, therefore, exceeds the release threshold and a release event isdeemed to have occurred at sample 101 and not earlier. At sample 102,the transformed value begins to increase. At sample 105, the transformedvalue goes positive, and the integration is reset to zero. (The releaseillustrated in FIG. 18 is a relatively abrupt, and not a slow, release.The foregoing principles, however, could be used to detect a slowrelease. Similarly, these principles also could be used to detect anabrupt touch.)

The principles of the disclosure could be applied to determine touchbased on noise. For example, a current system noise level may bedetermined by summing the absolute values of bins 4, 5, 6, and 7 for agiven sample and the previous sample. This noise level determination isdynamic in that it is recalculated as the eight rolling sample indexes,as discussed above. According to an eighth method, a touch may be deemedto have occurred when the value of bin 1 reaches a positive peak greaterthan a predetermined threshold that may be calculated as a function ofthe current noise level. For example, the threshold may be calculated bymultiplying the current noise level by a value greater or less than 1.In an embodiment, this multiplier may have a value of ⅔. A release maybe deemed to have occurred when the value of bin 1 reaches negative peakgreater than the same or another predetermined threshold that may becalculated in a similar manner. These situations are illustrated in FIG.19.

According to a ninth method, the foregoing noise threshold technique maybe used in combination with the integration technique discussed above.Again, a current noise level may be determined by summing the absolutevalues of bins 4, 5, 6, and 7 for a given sample and the previoussample. A touch may be deemed to have occurred when the positiveintegration of the values in bin 1 between points having zero ornegative values is greater than a predetermined threshold that may becalculated as a function of the current noise level, for example, amultiple or ratio of the current noise level. A release may be deemed tohave occurred when the negative integration of the values in bin 1between points having zero or positive values is greater than apredetermined threshold that may be calculated as a function of thecurrent noise level, for example, a multiple or ratio of the currentnoise level. These situations are illustrated in FIG. 20.

According to a tenth method, the dynamic noise level itself may be usedto determine touch based on a comparison of the current noise level tothe previous noise level. Noise typically is at a minimum or relativelylow in the untouched state and the hold state. Noise increases inresponse to touch and also in response to a release. Noise associatedwith a release typically is greater than noise associated with a touch.A touch may be deemed to have occurred when the noise level increasesabove a predetermined threshold. A release may be deemed to haveoccurred when the noise level drops or by a second increase in noiselevel, possibly greater than the noise level indicative of the touch(because releases generally involve higher noise levels than touches).These situations are illustrated in FIG. 21, where the dips in the noisecurve between the touch and release points correspond to momentaryholds.

Although the foregoing method could be used to determine touch andrelease for quick taps (quick touch and release with no or minimalhold), it could yield ambiguous results if applied to long key presses(touch followed by extended hold followed by release). As set forthabove, the dynamic noise level is a summation of absolute values and,therefore, always is zero or positive. Also, both touch and release areaccompanied by an increase in noise and noise is at a minimum orrelatively low during the untouched state and the hold state. Thesephenomena are illustrated in FIG. 22, wherein touch and release appearsimilar and may be confused. According to an eleventh method, the sign(positive or negative) of the signal in bin 1 may be used betterdiscriminate between touch and release. A touch may be deemed to haveoccurred when the noise level increases and the sign of the signalcomponent in bin 1 is positive. Similarly, a release may be deemed tohave occurred when the noise level increases and the sign of the signalcomponent in bin 1 is negative.

The foregoing describes illustrative methods of determining touch and/orrelease. Other methods could be used, as well. Also, touch and/orrelease could be determined using any combination of the foregoingand/or other methods. For example, one method could be used to determinewhether or not a touch or release has occurred and another method couldbe used to confirm that the touch or release has occurred. Further, thecombination of methods could include use of a time domain or otherconventional technique for determining touch and/or release. Forexample, a frequency domain or sequency domain technique could be usedto determine whether or not a touch or release has occurred and a timedomain technique could be used to confirm that the touch or release hasoccurred. Using a combination of methods or techniques to determinetouch and release could yield improved accuracy and/or reliability.

The Walsh-Hadamard transform described herein involves multiplication ofan eight point rolling sample of capacitive signals by an 8×8 Walshmatrix, yielding transformed signals in eight signal bins. In otherembodiments, the Walsh-Hadamard transform could be performed on a longeror shorter rolling sample of raw data by application of acorrespondingly-sized Walsh matrix on the sample. For example, theWalsh-Hadamard transform could be performed by applying a 4×4 Walshmatrix to a 4 point rolling sample, or it could be performed by applyinga 16×16 Walsh matrix to a 16 point rolling sample. In other embodiments,a Walsh matrix of any size could be applied to a correspondingly sizedrolling sample. As such, the Walsh-Hadamard transform is scalable.

In general, smaller transforms require less computation, but yield fewersequency bins. Similarly, larger transforms yield more sequency bins,but also require more computation. More sequency bins may be moredesirable in certain applications because they provide more data (ordata in more forms).

The data in certain sequency bins might be more useful than the data inothers. Indeed, the data in some sequency bins might not be particularlyuseful in a particular application. For example, the data in somesequency bins may be indicative only or predominantly of noise, whichtypically is not useful in determining whether a touch event hasoccurred. As such, it might not be necessary to calculate a value foreach sequency bin when performing the Walsh-Hadamard transform. Instead,it might be sufficient to identify the sequency bins expected to containthe data necessary or desirable for a particular application, to computethe data only for those bins, and to not compute the data that would becontained in other bins. Performing the Walsh-Hadamard transformselectively in this manner could significantly reduce the load on theprocessor used to perform the corresponding computations. For example,if it is known that the data in sequency bins 0-5 will be used indetermining whether a touch event has occurred and the data in sequencybins 6 and 7 will not be used, the values for sequency bins 6 and 7 neednot be calculated. Selectively omitting calculations in this manner mayconsiderably reduce the demand on the processor and save considerableprocessing time.

The present disclosure has thus far described use of a Walsh-HadamardTransform to transform capacitive signals from the time domain to thesequency domain. In alternative embodiments, a Fourier Transform or FastFourier Transform (FFT) could be used in a similar manner to transformcapacitive signals from the time domain to the frequency domain, aswould be understood by one skilled in the art. In further embodiments,other types of transforms could be used in a similar manner to transformcapacitive signals from the time domain to the frequency or sequencydomains, as would be understood by one skilled in the art.

For example, the principles of the disclosure could be applied toembodiments using an FFT. FIGS. 23-30 illustrate distribution curves foreight frequency bins in the frequency domain resulting from applicationof an 8×8 FFT to the time domain data of FIG. 3. More specifically, FIG.23 illustrates frequency distribution curves for the real data infrequency bins 0-7. FIG. 24 illustrates frequency distribution curvesfor the real data in frequency bins 1-3. FIG. 25 illustrates frequencydistribution curves for the real data in frequency bins 1-3 andidentifies touched and untouched states. FIG. 26 illustrates frequencydistribution curves for the imaginary data in frequency bins 1-3. FIG.27 illustrates frequency distribution curves for the imaginary data infrequency bins 1-7. FIG. 28 illustrates frequency distribution curvesfor the magnitude data in frequency bins 1-3. FIG. 29 illustratesfrequency distribution curves for the magnitude data in frequency bins1-7. The principles of the disclosure could be used to determine touchand release events using the frequency domain data resulting fromapplication of the FFT in a manner analogous to that described above inconnection with the sequency domain data.

The disclosure thus far has illustrated and described applicationsinvolving a single electrode touch sensor and principles ofself-capacitance. The principles of the disclosure could be applied toother electrode structures, as well. For example, the principles of thedisclosure could be applied to electrode structures including first andsecond, or drive and sense electrodes and involving principles of mutualcapacitance. Such sensors could be discrete sensors or portions of atouch screen having plural drive and sense lines.

FIG. 30 illustrates an embodiment including four spaced apart x (ordrive) electrodes D1-D4 and a single y (or sense) electrode S configuredas four touch detection zones T1-T4. This electrode structure could beused, for example, as a slider and/or as four discrete keys.

FIG. 31 illustrates an embodiment including four dedicated x or driveelectrodes D1-D4 and a single y or sense electrode S configured as asingle touch detection zone T. This electrode structure could be used,for example, as a hand sensor for an automobile door handle.

The invention claimed is:
 1. A method for operating a capacitive sensor,comprising: providing a sensor electrode; periodically exciting thesensor electrode and thereby periodically generating an electric fieldabout the sensor electrode and a capacitance between the sensorelectrode and a reference potential; periodically determining in thetime domain values of capacitance between the sensor electrode and thereference potential; transforming a rolling sample of theperiodically-determined values of capacitance from the time domain to aplurality of data signals in the sequency domain using a Walsh-HadamardTransform, each of the data signals in a corresponding sequency binresulting from the Walsh-Hadamard Transform and each of the data signalscomprising values related to capacitance for corresponding samples ofthe rolling sample; analyzing in the sequency domain at least one of theplurality of data signals in the corresponding sequency bin; anddetermining whether a touch event has occurred based on the analysis ofthe at least one of the plurality of data signals in the correspondingsequency bin; wherein the rolling sample is an n-point rolling sampleincluding a current sample and n−1 samples immediately preceding thecurrent sample, wherein the n−1 samples includes a last sample, andwherein n is an integer greater than or equal to 2 corresponding to asize of a Walsh matrix used in performance of the Walsh-HadamardTransform.
 2. The method of claim 1 wherein n equals
 8. 3. The method ofclaim 1 wherein a touch event is deemed to have occurred when thedifference between a baseline value and the value of the current samplein a sequency bin 0 exceeds a predetermined threshold.
 4. The method ofclaim 1 wherein a touch event is deemed to have occurred when the valueof the last sample in a sequency bin 1 is greater than the value of thecurrent sample in the sequency bin
 1. 5. The method of claim 4 wherein arelease is deemed to have occurred when the value of the current samplein the sequency bin 1 reaches a minimum.
 6. The method of claim 1wherein a touch event is deemed to have occurred when the differencebetween the value of the current sample in a sequency bin 1 and thevalue of the previous sample in the sequency bin 1 exceeds apredetermined threshold.
 7. The method of claim 1 wherein n equals atleast 8 and wherein a touch event is deemed to have occurred when thedata signals in sequency bins 1, 2, 3, 6 and 7 are greater than zero andthe data signals in sequency bins 4 and 5 are less than zero.
 8. Themethod of claim 1 wherein a touch event is deemed to have occurred whenthe data signal in a sequency bin 1 crosses zero in the positivedirection and then crosses zero in the negative direction.
 9. The methodof claim 1 wherein a touch event is deemed to have occurred whenintegration of successive non-negative transformed values of the samplesin a sequency bin 1 yields a value that meets or exceeds a predeterminedthreshold.
 10. The method of claim 1 wherein a release event is deemedto have occurred when the absolute value of the value yielded byintegration of successive non-positive transformed values of the samplesin a sequency bin 1 meets or exceeds a predetermined threshold.
 11. Themethod of claim 1 wherein a touch event is deemed to have occurred whenthe sum of the values of the samples in a sequency bin 1 exceeds asystem noise level by a predetermined multiplier.
 12. The method ofclaim 11 wherein the multiplier is less than one.
 13. The method ofclaim 11 wherein the system noise level is determined by summing theabsolute values of the samples in sequency bins 4, 5, 6, and
 7. 14. Themethod of claim 1 wherein a touch event is deemed to have occurred whenthe absolute value of the value yielded by integration of successivenon-positive transformed values of the samples in a sequency bin 1 meetsor exceeds a predetermined threshold that is a function of a systemnoise level.
 15. The method of claim 1 wherein a touch event is deemedto have occurred when a system noise level increases above apredetermined threshold.
 16. The method of claim 1 wherein theWalsh-Hadamard transform is a Fast Walsh-Hadamard transform.
 17. Themethod of claim 1 wherein the values related to capacitance comprisecounts.